Electrostatic thrustor with improved insulators



June 13, 1967 BYERS ET AL 1 3,324,659 ELECTROSTATIC THRUTOR WITH IMPROVED INSULATORS Filed Sept. 7, 1965 WI UHIIL m l lmw I/Il/I/IIlII/IIIlI/II INVENTORS DAVID C. BYERS PAUL M. MARGOSIAN GLENN E. MEALY United States Patent ABSTRACT OF THE DISCLOSURE High voltage insulators for direct current in a vacuum environment. Electrons released by field emission are constrained in the region of the negative junction to eliminate surface flashover and unstable operation. The insulators are particularly useful in the acceleration system of an electrostatic thrustor.

The invention described herein may be manufactured and used by and for the Government of the United States of America for governmental purposes without the payments of any royalties thereon or therefor.

This invention is concerned with an improvement in high voltage insulation in a vacuum and, more particular- 1y, with insulators for an electrostatic thrustor. The invention is primarily directed to improved insulators for use in the acceleration system of an electron bombardment ion thrustor.

High voltage direct current insulation in a vacuum environment has been a problem in the field of electrostatic thrustors. Spherical insulators of the type described in U.S. Patent No. 3,156,090 do not withstand the voltage required for the operation of high impulse thrustors which may utilize up to 100,000 volts in their acceleration systems. It has been found that a spherical insulator having a diameter of two inches usually breaks down at about 22,000 volts.

A proposed solution to this problem has been to increase the size of the spherical insulators. Such an eX- pedient is undesirable in space applications where an electrostatic thrustor is used because of weight restriction.

This insulation problem arises because electrical breakdown across an insulator usually commences in a vacuum with electrons drawn from the negative elect-rode striking the positive electrode and knocking out more electrons. This cascade or avalanche process is the most widely held theory of vacuum breakdown.

With the spherical insulators of the type shown in the aforementioned US. patent, high stress fields are prevalent at the points of contact between the spheres and the planes or spaced electrodes because of the small area of contact. In such installations where strong electric fields are prevalent, as in the case of an electrostatic thrustor, a strong electric field at a-point on the negative electrode will extract electrons by field emission.

The problems arising from the use of conventional insulators for electrostatic thrustors have been solved by utilizing insulators constructed in accordance with the present invention. Because the electrostatic field is directed through the dielectric material of these insulators, electrons emitted by field emission are constrained in the region of the negative junction. Also, there is probably some reduction in the electric field strength at the negative junction. In this manner, surface flashover and unstable operation at high voltages have been eliminated, and the maximum voltage the insulators of the invention can support is limited only by the dielectric strength of the insulator material.

It is, therefore, an object of the present invention to provide an improved high voltage insulator for use in a vacuum environment.

Another object of the present invention is to provide an improved insulator for an electrostatic ion thrustor.

These and other objects of the invention will be apparent from the specification which follows and the drawings wherein like numerals are used throughout to identify like parts. The drawing is a view in elevation with parts in section of an electrostatic thrustor having an acceleration system which utilizes insulators constructed in accordance with the present invention.

Referring now to the drawing there is shown an electrostatic thrustor 10 having an acceleration system which utilizes insulators 12 constructed in accordance with the present invention. The thrustor 10 includes a cylindrical casing 14, the center of which forms an ion chamber. A propellant source 16 is located at the forward end of the thrustor 10 and is in communication with the ion chamber within the casing 14. This propellant source may be in the form of a steam jacketed boiler of the type shown in Patent No. 3,156,090 having a calibrated orifice at the entrance to the ion chamber.

Neutral particles in the form of a gaseous propellant, such as mercury, enter the upstream end of the ion chamber in which high velocity electrons thermionically emitted by a cathode in the form of a hot filament in the chamber ionize these particles to form a plasma. A screen grid 18 and an accelerator grid 20 at the downstream end of the ion chamber comprise the accelerator system of the thrustor. This accelerator system serves to focus and accelerate ions that reach the downstreamend of the thrustor. The thrust producing mechanism of this device is the momentum change of the ions as they are accelerated by an electrostatic field which is applied between the screen and accelerator grids.

Both the screen grid 18 and accelerator grid 20 are connected to suitable sources of power in the manner shown in the aforementioned Patent No. 3,156,090. The screen grid 18 is maintained at the voltage potential substantially equal to that of the thermionically emitting cathode within the ion chamber while the accelerator grid 20 has relatively high negative potential relative to the screen grid.

The accelerator grid 20 is mounted on the screen grid 18 by brackets 30 which serve to maintain the proper spacing between these two grids, as shown in the drawing. Each bracket 30 has a J-shaped configuration and is secured at one end 32 to the accelerator grid 20. Each bracket 30 curves toward the forward end of the thrustor 10 and has a head 34 mounted on the opposite end. A pin 36 extends through the head 34 and is substantially parallel with the axis of the thrustor 10.

The screen grid 18 carries spaced arms 40 which extend radially outward therefrom. A plate 42 is mounted on each of the arms 40 by spaced bolts 44.

The insulators 12, which are constructed in accordance with the present invention, are mounted between the arms 40 and the plates 42 as shown in the drawing. Each insulator 12 is of a dielectric material, such as aluminum oxide, and has a substantially flat face which engages either the arm 40 or the plate 42.

At the opposite end is the recessed portion 50 which receives the pin 36 shown in the drawing. It has been found that the pin 36 should extend into the recessed portion of the insulator 12 a distance greater than the width of the opening of the recessed portion.

As shown in the drawing, the recessed portion 50 is provided with a cylindrical depression 52 at the bottom thereof. This depression serves to decrease the local electric field strength at the junction between the pin 36 and the insulator 12. It has been found that the ratio of the penetration of the pin 36 in the depression 52 to the gap between the pin 36 and the surface of the depression 52 should be about 4 .to 1. While further penetration of the pin 36 into the depression 52 provides some improvement in performance, this improvement is slight when the aforementioned ratio is exceeded.

The accelerator grid 20, as well as the bracket 30 with the pin 36 connected thereto, is highly negative relative to the screen grid 18 and the arms 40 as previously explained. Consequently, each pin 36 constitutes a negative electrode. The recessed portion 52 of each insulator 12 serves to shield this negative electrode to prevent electrical breakdown at high voltages.

While the preferred embodiment of the invention has been shown and described, it will be appreciated that various structural modifications can be made to the insulator as well as the electrostatic thrustor without departing from the spirit of the invention or the scope of the subjoined claims. For example, it is contemplated that insulators constructed in accordance with the invention could be utilized for high voltage AC vacuum applications by mounting the insulators in a back-to-back fashion.

What is claimed is:

1. In an electrostatic ion thrustor of the type having a casing forming an ion chamber with a screen grid adjacent one end thereof and an accelerator grid spaced therefrom having a high negative DC potential relative to the screen grid and casing to accelerate ions away from the thrustor in a vacuum environment, the improvement comprising a plurality of insulators for electrically insulating the accelerator grid from the casing and screen grid, each of said insulators having a recessed portion,

a plurality of elongated members rigidly mounted on the accelerator .grid, each of said elongated members having the same high negative potential relative to the screen grid thereby forming a negative electrode, and

means for rigidly mounting said insulators on the casing adjacent said elongated members with the openings of the recessed portions facing said elongated members for receiving the same and shielding the ends thereof to prevent electrical breakdown at high voltages.

2. Apparatus as claimed in claim 1 wherein the elongated members extend into the recessed portion of the insulator a distance greater than the width of the opening of the recessed portion. 7

3. Apparatus as claimed in claim 1 wherein the insulator has a depression at the end of the recessed portion remote from the opening thereof for contacting the outermost end of the elongated members.

4. Apparatus as claimed in claim 3' wherein the width of the depression is greater than that of the elongated members to form a gap between the surfaces of the depression and elongated members.

5. Apparatus as claimed in claim 4 where the ratio penetration of the elongated members into the depression to the gap is about 4 to 1.

6. An acceleration system for an electrostatic ion thrustor of the type having a casing forming an ion chamber comprising a screen grid mounted on the casing at one end of the ion chamber and having the same potential as the casing,

a plurality of spaced arms carried by said screen grid and extending radially outward from the casing,

a plate mounted on each of said arms and spaced therefrom,

an accelerator grid spaced from said screen grid and having a high negative potential relative to the casa plurality of brackets, each having one end secured to said accelerator grid and another end positioned in the space between one of said arms and the plate mounted thereon,

a pin mounted on said other end of each of said brackets in said space, said pin having one end facing said one of said arms and another end facing said plate,

a first insulator having a flat surface in engagement with said one of said arms and a concave surface adjacent said one end of said pin, and

a second insulator having a flat surface in engagement with said plate mounted on said one of said arms and a concave surface adjacent said other end of said pin.

7. An accelerator in claim 6 including a depression in the first insulator for receiving the one end of the pin, and

a depression in the second insulator for receiving the other end of the pin.

system for an ion thrustor as claimed References Cited UNITED STATES PATENTS 861,275 7/1907 Gallagher l74-174 X 885,678 4/1908 Steinberger 174-174 X 2,191,152 2/ 1940 Hammel 174-474 X 2,379,422 7/1945 Busignies et al. 174-438 X 3,104,279 9/1963 Meier l74-212 X 3,194,879, 7/1965 Hopwood 17421'2 X 3,262,262 7/ 1966 Reader et al 31363 X CARLTON R. CROYLE, Primary Examiner. 

6. AN ACCELERATION SYSTEM FOR AN ELECTROSTATIC ION THRUSTOR OF THE TYPE HAVING A CASING FORMING AN ION CHAMBER COMPRISING A SCREEN GRID MOUNTED ON THE CASING AT ONE END OF THE ION CHAMBER AND HAVING THE SAME POTENTIAL AS THE CASING, A PLURALITY OF SPACED ARMS CARRIED BY SAID SCREEN GRID AND EXTENDING RADIALLY OUTWARD FROM THE CASING, A PLATE MOUNTED ON EACH OF SAID ARMS AND SPACED THEREFROM, AN ACCELERATOR GRID SPACED FROM SAID SCREEN GRID AND HAVING A HIGH NEGATIVE POTENTIAL RELATIVE TO THE CASING, A PLURALITY OF BRACKETS, EACH HAVING ONE END SECURED TO SAID ACCELERATOR GRID AND ANOTHER END POSITIONED IN THE SPACE BETWEEN ONE OF SAID ARMS AND THE PLATE MOUNTED THEREON, A PIN MOUNTED ON SAID OTHER END OF EACH OF SAID BRACKETS IN SAID SPACE, SAID PIN HAVING ONE END FACING SAID ONE OF SAID ARMS AND ANOTHER END FACING SAID PLATE, A FIRST INSULATOR HAVING A FLAT SURFACE IN ENGAGEMENT WITH SAID ONE OF SAID ARMS AND A CONCAVE SURFACE ADJACENT SAID ONE END OF SAID PIN, AND A SECOND INSULATOR HAVING A FLAT SURFACE IN ENGAGEMENT WITH SAID PLATE MOUNTED ON SAID ONE OF SAID ARMS AND A CONCAVE SURFACE ADJACENT SAID OTHER END OF SAID PIN. 